Structure, synthèse et dégradation des chitinoprotéines de la cuticule des Crustacés décapodes

The procuticle of the Crustacea Decapoda is formed of stacked laminae composed of fibre beds in which the fibres run parallel to the surface and undergo a slight and progressive change in orientation from one bed to another. The basic structural unit of the procuticle is a microfibril made up of a core of chitin criystallite surrounded by a protein sheath. Microfibrils arise at the surface of plasmamembrane plaques which are concerned with their deposition and orientation. According to the degree of mineralization, microfibrils are either homogeneously distributed (membranous layer) or secondarily associated into reticulate structures (pigmented layer) or into macrofibres (principal layer). In the newly secreted procuticular sheets the chitin-proteic fibres are disposed around long vertical microvilli originating from the epidermis. The plasmamembrane of the microvilli appears to be closely concerned with the deposition of fibrous components running vertically. Progressively the microvilli degenerate and leave hollow twisted tubes, the cuticular canals (pore canals), delimited by a helical fibre sheath. These canals may project lateral connections and constitute routes through which material involved in cuticle hardening can gain access to the external laminae. The membranous layer, deposited
a long time after ecdysis, is not mineralized and exhibits a peculiar chemical composition as well as hygroscopic properties. It keeps hydrolytic enzymes in store during the preecdysial period and plays the role of lubrificating agent at the critical moment of ecdydis. The enzymatic degradation of the old cuticle during the preecdysial period is characterized by an important decrease of chitin and soluble protein contents. In contrast, the cuticular calcium is not reabsorbed, at least in Cancer pagurus and Carcinus maenas.